Hydrogen sulfide is an abundant material today as the by-product from hydrodesulfurization of oil and coal. Chemical synthesis processes employing available hydrogen sulfide as an ingredient are very desirable. It is known that hydrogen sulfide can act as a reducing agent, but, it is much less active than for instance native hydrogen. Therefore, so far as we are aware, the application of hydrogen sulfide has been limited in the past to specific reactions whereas other reactions have not been possible to carry out. For instance U.S. Pat. No. 3,994,980 to Phillips Petroleum Company discloses a process for preparing thiols by contacting a mixture of a carbonyl group containing compound with a sulfur source such as hydrogen sulfide for obtaining thiols. We are unaware of any prior use of hydrogen sulfide for reduction of organic sulfonyl halides. Wilbur A. Lazier et al. in U.S. Pat. No. 2,402,641 discloses the preparation of aryl thiols by hydrogenation of sulfonic acids and of compounds hydrolyzable to these acids with elemental hydrogen.
The preparation of sulfonyl halides is well known for aromatic compounds since the beginnings of the dye and pigment industry.
The selective reduction of aromatic sulfur compounds is frequently difficult and expensive. Direct hydrogenation of an aromatic sulfide, sulfonyl halide or thiol will generally result in cleavage of the sulfur carbon bond to yield hydrogen sulfide and the aromatic group. Presently, the reduction of 2,5-dichlorobenzyl sulfonyl halide is carried out commercially with zinc metal in order to obtain 2,5-dichlorobenzene thiol. In addition to reduction of arene sulfonyl chlorides by various metals, electrolysis is used for such reduction. For instance, benzene sulfonyl chlorides have been reduced with iron and with electrochemical reduction. U.S. Pat. No. 2,820,780 indicates the possibility of reducing organic sulfides to thiols with hydrogen sulfide. Specific examples disclose the reduction of cystline to cysteine and of 2,2-dihydroxy 6,6-dinaphtyl disulfide to two thiol molecules and of glutathione [GSSG] to two thiol molecules [GS].